In a method for fabricating a semiconductor device, in processes such as a process of diffusion of impurities, of forming insulating layers, surface treatments by etching have been employed. According to these processes, on a surface of a semiconductor body, insulating layers having protuberances and/or step-like cross-sections (hereafter, these are called uneven surface) are formed. That is, a surface of a substrate which includes insulating layers, wirings, etc., and on which other components for the semiconductor device are to be formed, is an uneven surface. For example, when wirings are formed on the surface of the body, the surface of the substrate becomes uneven. Therefore, when multilevel interconnections, for example, are formed on the uneven surface of the substrate, the multilevel interconnections to be formed also have an uneven surface due to the uneven surface of the substrate, and hence the more external is the interconnection, the more non-uniform the thickness of the interconnection becomes. When the non-uniformity becomes extreme, the interconnection often breaks at a certain portion. This defect lowers the reliability of the semiconductor devices. For improving the reliability of the semiconductor devices, thicker interconnections are generally utilized.